10
SMT Assembly
passes through one or more
Assembly Information
SOT-323 PCB Footprint
Reliable assembly of surface
mount components is a complex
process that involves many
material, process, and equipment
factors, including: method of
heating (e.g., IR or vapor phase
reflow, wave soldering, etc.)
circuit board material, conductor
thickness and pattern, type of
solder alloy, and the thermal
conductivity and thermal mass of
components. Components with a
low mass, such as the SOT-323
package, will reach solder reflow
temperatures faster than those
with a greater mass.
preheat zones. The preheat zones
increase the temperature of the
board and components to prevent
thermal shock and begin evaporat-
ing solvents from the solder paste.
The reflow zone briefly elevates
the temperature sufficiently to
produce a reflow of the solder.
A recommended PCB pad layout
for the miniature SOT-323 (SC-70)
package is shown in Figure 27
(dimensions are in inches). This
layout provides ample allowance
for package placement by auto-
mated assembly equipment
without adding parasitics that
could impair performance.
The rates of change of tempera-
ture for the ramp-up and cool-
down zones are chosen to be low
enough to not cause deformation
of the board or damage to compo-
nents due to thermal shock. The
maximum temperature in the
0.026
0.07
reflow zone (T
exceed 235 °C.
) should not
MAX
0.035
HP’s SOT-323 diodes have been
qualified to the time-temperature
profile shown in Figure 28. This
profile is representative of an IR
reflow type of surface mount
assembly process.
These parameters are typical for a
surface mount assembly process
for HP SOT-323 diodes. As a
general guideline, the circuit
board and components should be
exposed only to the minimum
temperatures and times necessary
to achieve a uniform reflow of
solder.
0.016
Figure 27. PCB Pad Layout
( dimensions in inches) .
After ramping up from room
temperature, the circuit board
with components attached to it
(held in place with solder paste)
250
200
TMAX
150
Reflow
Zone
100
Preheat
Zone
Cool Down
Zone
50
0
0
60
120
180
240
300
TIME (seconds)
Figure 28. Surface Mount Assembly Profile.
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
